The present invention relates to the purification of aromatic ketones. More particularly, the present invention relates to the purification of acetophenone containing difficultly separable hydrocarbon impurities.
Acetophenone is a by-product of various known processes for the preparation of styrene by the dehydration of alpha-methylbenzyl alcohol, when ethylbenzene is employed as the alcohol precursor. Such dehydration processes are exemplified by the liquid phase aralkanol dehydration process disclosed in U.S. Pat. No. 3,526,674. The net product from the reaction system disclosed in the '674 Patent comprises styrene, acetophenone and alpha-methyl-benzyl alcohol and minor amounts of other impurities (e.g., see Example VI of the '674 Patent). Styrene product is separable from this net reaction system product by fractional distillation, as is a by-product comprising alpha-methylbenzyl alcohol and acetophenone. The acetophenone content of this by-product stream may range broadly from about 30 to 80 wt. %.
By subjecting the by-product to catalytic hydrogenation, acetophenone may be converted to alpha-methylbenzyl alcohol and the hydrogenate may be returned to the dehydration zone to produce additional styrene product. For example, see U.S. Pat. No. 3,927,120.
Acetophenone by-product streams would also appear to be a valuable source of high-purity (98 wt. %+) acetophenone product. For example, light impurities such as styrene, alpha-methylstyrene, cumene, benzaldehyde and ethylbenzene could be removed overhead in a first fractional distillation and heavy impurities, including alpha-methylbenzyl alcohol could be removed in the bottoms of a second fractional distillation. However, it has been found that such fractional distillation of acetophenone by-product streams recovered from liquid phase dehydration processes operating according to the teachings of the '674 patent yield a final product containing only about 97 wt. % acetophenone.
The known impurities which were the keys in the design of the conventional separation scheme--benzaldehyde (light key) and alpha-methylbenzyl alcohol (heavy key)--were completely separated. However, the presence of certain other (previously unknown) impurities makes it impracticable to obtain better than 97 wt. % purity acteophenone by the two conventional distillation steps. The previously unknown impurities have been identified as 1,2,4-trimethylbenzene; 1-methyl tetralin; an alkylated benzyl alcohol (164 molecular weight); and paraffinic hydrocarbons (156 average molecular weight).
U.S. Pat. No. 3,819,492 discloses the purification of aromatic ketones containing difficulty separable acids and alcohols by distillation in the presence of an acid capable of catalyzing esterification between the acid and alcohol impurities. Such impurities are then separable by conventional distillation.
An object of the present invention is a method for the purification of acetophenone containing difficulty separable hydrocarbon impurities. A related object is the recovery of high-purity acetophenone from byproduct streams produced in the production of styrene by the dehydration of alpha-methylbenzyl alcohol, especially byproduct streams produced in liquid-phase dehydration processes.